1. Field of the Invention
The present invention generally relates to communication systems and in particular multiple antenna systems.
2. Description of the Related Art
A basic arrangement for communication systems is to have at least one stream transmitting signals to at least one receiver over a communication channel. A key goal in communication systems is to efficiently manage the resources available at the transmitter and receiver. For the stream it is desirable to transmit the communication signals at as high a data rate as the system will allow. Usually the factor that limits the transmission rate is the amount of transmission power that is allocated to the transmitter. Also, because the communication channel is not ideal, errors will be introduced into the signals as they propagate through the communication channel. The communication channel is typically a dynamically time varying system that affects communication signals differently at different times. For the receiver it is desirable to receive the signals with as little error as possible. To reduce the errors as much as possible, the receiver is often designed such that it sends feedback information to the transmitter characterizing the communication channel. The transmitter uses the feedback information to modify its transmit signals to reduce their vulnerability to the anomalies of the communication channel. The feedback information transmitted by the receiver, however, represents additional bandwidth (another system resource) that the system sometimes uses to increase its efficiency. Communication system designers have discovered that if they can determine the characteristic of the channel from the transmitter""s point of view, they can allocate the proper amount of data rate or bandwidth and power to the transmitter. A transmitter, which xe2x80x98seesxe2x80x99 a certain channel characteristic, can handle a xe2x80x98certain data rate.xe2x80x99 There is a direct relationship between the channel characteristic and the proper amount of resource allocation (e.g., data rate) such a channel characteristic can handle. Thus, allocating a higher data rate than the xe2x80x98certain data ratexe2x80x99 to that transmitter will cause a portion of the data transmitted by that transmitter to be lost because, based on the channel characteristic, the channel cannot handle the additional data rate. Since no data could be correctly received during this time, the average data rate (defined as the total data correctly received divided by the total time of transmission) becomes smaller, thus lowering the efficiency of the communication system. Conversely, if the transmitter is allocated a lower data rate than the rate it can handle, the transmitter would not be operating efficiently since it can transmit more information that can be correctly received; this is not an efficient use of the available data rate or bandwidth. If, however, the channel characteristic as seen by the transmitter can be determined, then the proper amount of bandwidth and power (or other resources) can be assigned to the transmitter allowing it to operate efficiently. The channel characteristic represents how the channel affects the parameters (e.g., amplitude, phase, frequency) of a transmitted signal. The channel characteristic is typically obtained from measurement of signals after they have propagated through the channel. Because the parameters of the signal are known prior to them being transmitted through the channel and further because the parameters of the signals can be measured after they have propagated through the channel, the effects of the channel on the signals can be easily determined at the receiver and this information can be conveyed back to the transmitter.
In multiple transmitter communication systems with one or more transmitters there could be a plurality of data streams (referred to as stream hereafter) that are transmitted simultaneously. The streams can be part of a transmitted signal or a plurality of transmitted signals from the transmitters. The goal remains to determine the channel characteristic seen by each stream so that the proper amount of one or more resources can be allocated to each stream. System designers use a particular well known filter at the receiver called a Minimum Mean Squared Error (MMSE) filter and determine the channel quality at the output of the MMSE filter to allow them to determine the channel characteristic as seen by each stream and then allocate the proper amount of one or more resources to each stream based on the determined channel characteristic. The receiver has one or more receiving devices (e.g., antennas) for receiving the signals from the streams. The received signal is processed as per the MMSE technique to determine the characteristic of each of the streams. However, even though the MMSE technique allows one to determine the channel characteristic of each of the streams, the MMSE receiver does not provide relatively good performance when compared to APP (A Posteriori Probability) receiver. APP receiver provides better performance than MMSE receiver in that they are able to receive information at a relatively lower error rate than MMSE receivers. However, unlike MMSE, determining the channel characteristic as seen by each transmitter for APP receivers has been a difficult problem and has not been addressed before.
The present invention provides a method for efficiently allocating one or more resources to at least one stream of a communication system by performing an APP decoding process at a receiver of the communication system capable of receiving signals from the streams over a communication channel. We describe a process to determine the channel characteristic as seen by each stream thus enabling the proper amount of one or more resources to be allocated to each stream of the communication system. The invention determines the channel characteristic per stream by using Modified Union bounds.
A modified union bound for each stream is determined where each modified union bound is a function of the following three parameters: (1) the channel matrix as seen by the receiver; (2) the transmit power of the signals from the stream and (3) the variance of the noise in the communication channel assuming that the noise follows Gaussian statistics and has zero mean. A Q(x) function is used to determine the modified union bounds where x represents a combination of the three parameters. Modified union bounds provide an upper bound probability of error (Pe) value of any particular symbol transmitted for a particular stream. Modified union bounds are based on union bounds, which is a well known technique that is used in communication system determine an upper bound the probability of error for the collection of symbols transmitted simultaneously in the streams. A transmitted symbol of any particular stream is drawn from a constellation of symbols. A probability of symbol error vs. SNR (Signal to noise ratio) curve is generated for the symbol constellation of a particular stream assuming an Additive White Gaussian Noise (AWGN) channel. SNR is the ratio of the signal power to the noise power. Probability of error versus SNR curves can be obtained by well-known numerical methods and/or statistics. The calculated upper bound probability of error value (Pe) using modified union bounds is applied to the probability of symbol error vs. SNR curve to obtain an SNR value. This SNR value is used to determine a proper data rate for that stream where the data rate is one or more resources that desirably are to be efficiently allocated amongst the streams. The system can choose to transmit the using a set of symbols in accordance with any one of various members of a Modulation and Coding Set (MCS). Each member of the MCS has associated with it a modulation scheme and an error correcting coding scheme. In essence, each member of the MCS defines a particular data rate. Further, each member of the MCS is associated with or can generate a particular set of coded symbols or codewords. A probability of codeword error vs. SNR curve for AWGN channel is generated yielding a set of such curves. The SNR value calculated from the probability of symbol error vs. SNR is applied to the set of curves (probability of codeword error vs. SNR) and the MCS that gives the highest data rate while keeping the probability of error small is chosen for that particular stream. The above process can be repeated for each of the streams allowing the data rate to be calculated for each stream with the APP decoding. Therefore, the appropriate data rate (or other resource) for any stream can be individually determined allowing efficient use of communication system resources.